Such a connecting circuit is, for example, known from the French Patent Application No. 7731145. The amplifiers are there used to feed the subscriber's line with direct current signals (DC-supply, polarity reversal) and alternating current signals (ringing). These amplifiers can also be used to apply the speech signals to the subscriber's line.
It is an object of the invention to derive signals from the telephone line in a manner which can withstand longitudinal currents flowing in the telephone line. These derived signals can be used for a plurality of purposes, for example to obtain (a) an output signal which is proportional to th loop current and independent of the longitudinal current in the telephone line for providing an output for the speech signal and an output for the loop state; (b) an output signal which is proportional to the longitudinal current and independent of the loop current and (c) feedback signals for the amplifier for simulating certain impedances which can withstand longitudinal currents through the outputs of the fed back amplifiers.